In an electro-hydraulic control system, an electronic controller controls an electric motor drivingly connected to a displaceable mechanical element which exerts force or pressure on hydraulic fluid. In turn, the hydraulic fluid pressure effectuates control of an output element of the hydraulic system. The electronic controller develops a motor control command for application to the electric motor to exert a force to achieve the desired control of the hydraulic system. The control command causes the electric motor to operate in a forward or a reverse direction at a specified torque. The control command includes a magnitude signal that varies in relation to the desired motor torque and a polarity signal that determines the direction of applied force.
In an ideal electro-hydraulic system, a direct relation exists between the control command and the desired effect on the mechanical element. The direct relationship between the control command and the resulting displacement of the mechanical element enables accurate pressure control for the hydraulic system. However, because of static friction of the mechanical element, it is difficult to actually achieve a direct relation between a given control command and displacement of the mechanical element. The static friction is the frictional force operating on the mechanical element to resist displacement from a resting position. Accordingly, if the mechanical element is at rest and the motor control command directs the motor to exert force, static friction may inhibit displacement of the mechanical element, making hydraulic pressure control difficult.
Electro-hydraulic control systems overcome such frictional forces using a variety of control methods. One method involves increasing the magnitude of a control command in successive steps until the force exerted by the electric motor overcomes the static friction. However, it is unknown exactly when static friction will be overcome and the mechanical element will begin to move. Because the magnitude of the control command is continually increased to effectuate displacement and the required displacement force is unknown, an imprecise relationship exists between control command magnitude and mechanical element displacement.
A second method of overcoming static friction involves establishing a base magnitude signal for a command in accordance with the desired hydraulic control, then substantially increasing that magnitude for a portion of the overall command duration. This temporary increase in magnitude occurs at the beginning of the command period, and is of a fixed magnitude and duration. Because the temporary magnitude increase must be of a duration greater than the response time of the mechanical element and the pressure-magnitude relationship is unknown, a direct relationship between the magnitude of the command and movement of the mechanical element remains difficult to establish.